This invention relates generally to observing fluorescence signals resulting from hybridization in conjunction with the polymerase chain reaction. More specifically, the present invention relates to observing hybridization with fluorescence during and/or immediately after PCR and using this information for product identification, sequence alteration detection, and quantification.
The polymerase chain reaction (PCR) is fundamental to molecular biology and is the first practical molecular technique for the clinical laboratory. Despite its usefulness and popularity, current understanding of PCR is not highly advanced. Adequate conditions for successful amplifications must be found by trial and error and optimization is empirical. Even those skilled in the art are required to utilize a powerful technique without a comprehensive or predictive theory of the process.
PCR is achieved by temperature cycling of the sample, causing DNA to denature (separate), specific primers to attach (anneal), and replication to occur (extend). One cycle of PCR is usually performed in 2 to 8 min, requiring 1 to 4 hours for a 30-cycle amplification. The sample temperature response in most PCR instrumentation is very slow compared to the times required for denaturation, annealing, and extension. The physical (denaturation and annealing) and enzymatic (extension) reactions in PCR occur very quickly. Amplification times for PCR can be reduced from hours to less than 15 min. Incorporated herein by reference in their entireties are each of the following individual applications, which disclose such a rapid cycling system: U.S. application Ser. No. 08/818,267, filed Mar. 17, 1997, entitled Method for Detecting the Factor V Leiden Mutation, which is a continuation-in-part of U.S. patent application Ser. No. 08/658,993, filed Jun. 4, 1996, entitled System And Method For Monitoring PCR Processes, which is a continuation-in-part of U.S. patent application Ser. No. 08/537,612, filed Oct. 2, 1995, entitled Method For Rapid Thermal Cycling of Biological Samples, which is a continuation-in-part of U.S. patent application Ser. No. 08/179,969, filed Jan. 10, 1994, (now U.S. Pat. No. 5,455,175), entitled Rapid Thermal Cycling Device, which is a continuation-in-part of U.S. patent application Ser. No. 07/815,966 filed Jan. 2, 1992, (now abandoned) entitled Rapid Thermal Cycling Device which is a continuation-in-part of U.S. patent application Ser. No. 07/534,029 filed Jun. 4, 1990, (now abandoned) entitled Automated Polymerase Chain Reaction Device. The copending U.S. application filed in the U.S. Patent and Trademark Office on Jun. 4, 1997, entitled System and Method for Carrying Out and Monitoring Biological Processes as Ser. No. 08/869,275 and naming Carl T. Wittwer, Kirk M. Ririe, Randy P. Rasmussen, and David R. Hillyard as applicants, is also hereby incorporated by reference in its entirety. Rapid cycling techniques are made possible by the rapid temperature response and temperature homogeneity possible for samples in high surface area-to-volume sample containers such as capillary tubes. For further information, see also: C. T. Wittwer, G. B. Reed, and K. M. Ririe, Rapid cycle DNA amplification, in K. B. Mullis, F. Ferre, and R. A. Gibb, The polymerase chain reaction, Birkhauser, Boston, 174-181, (1994). Improved temperature homogeneity allows the time and temperature requirements of PCR to be better defined and understood. Improved temperature homogeneity also increases the precision of any analytical technique used to monitor PCR during amplification.
Fluorimetry is a sensitive and versatile technique with many applications in molecular biology. Ethidium bromide has been used for many years to visualize the size distribution of nucleic acids separated by gel electrophoresis. The gel is usually transilluminated with ultraviolet light and the red fluorescence of double stranded nucleic acid observed. Specifically, ethidium bromide is commonly used to analyze the products of PCR after amplification is completed. Furthermore, EPA 0 640 828 A1 to Higuchi & Watson, hereby incorporated by reference, discloses using ethidium bromide during amplification to monitor the amount of double stranded DNA by measuring the fluorescence each cycle. The fluorescence intensity was noted to rise and fall inversely with temperature, was greatest at the annealing/extension temperature (50° C.), and least at the denaturation temperature (94° C.). Maximal fluorescence was acquired each cycle as a measure of DNA amount. The Higuchi & Watson application does not teach using fluorescence to monitor hybridization events, nor does it suggest acquiring fluorescence over different temperatures to follow the extent of hybridization. Moreover, Higuchi & Watson fails to teach or suggest using the temperature dependence of PCR product hybridization for identification or quantification of PCR products.
The Higuchi & Watson application, however, does mention using other fluorophores, including dual-labeled probe systems that generate fluorescence when hydrolyzed by the 5′-exonuclease activity of certain DNA polymerases, as disclosed in U.S. Pat. No. 5,210,015 to Gelfand et al. The fluorescence observed from these probes primarily depends on hydrolysis of the probe between its two fluorophores. The amount of PCR product is estimated by acquiring fluorescence once each cycle. Although hybridization of these probes appears necessary for hydrolysis to occur, the fluorescence signal primarily results from hydrolysis of the probes, not hybridization, wherein an oligonucleotide probe with fluorescent dyes at opposite ends thereof provides a quenched probe system useful for detecting PCR product and nucleic acid hybridization, K. J. Livak et al., 4 PCR Meth. Appl. 357-362 (1995). There is no suggestion of following the temperature dependence of probe hybridization with fluorescence to identify sequence alterations in PCR products.
The specific hybridization of nucleic acid to a complementary strand for identification has been exploited in many different formats. For example, after restriction enzyme digestion, genomic DNA can be size fractionated and hybridized to probes by Southern blotting. As another example, single base mutations can be detected by “dot blots” with allele-specific oligonucleotides. Usually, hybridization is performed for minutes to hours at a single temperature to achieve the necessary discrimination. Alternately, the extent of hybridization can be dynamically monitored while the temperature is changing by using fluorescence techniques. For example, fluorescence melting curves have been used to monitor hybridization. L. E. Morrison & L. M. Stols, Sensitive fluorescence-based thermodynamic and kinetic measurements of DNA hybridization in solution, 32 Biochemistry 3095-3104, 1993). The temperature scan rates are usually 10° C./hour or less, partly because of the high thermal mass of the fluorimeter cuvette.
Current methods for monitoring hybridization require a lot of time. If hybridization could be followed in seconds rather than hours, hybridization could be monitored during PCR amplification, even during rapid cycle PCR. The many uses of monitoring hybridization during PCR, as will be fully disclosed herein, include, product identification and quantification, sequence alteration detection, and automatic control of temperature cycling parameters by fluorescence feedback
The prior art, as explained above, carries out temperature cycling slowly and empirically. When analysis of PCR products by hybridization is needed, additional time consuming steps are required. Thus, it would be a great advance in the art to provide methods for monitoring hybridization during PCR and analyzing the reaction while it is taking place, that is, during or immediately after temperature cycling without manipulation of the sample. By monitoring hybridization during PCR, the underlying principles that allow PCR to work can be followed and used to analyze and optimize the reaction during amplification.